Panoramic versus Stereoscopic
Vision
The difference between panorama and stereo vision depends on how many optic nerve impulses are transferred
to the opposite side of the brain and how far the two eyes are displaced
to the sides of the head.
Total cross-over of the optic fibres gives panorama vision only.
Partial cross-over gives stereo plus limited panorama vision.
Primates
have the best stereo vision with 50% cross-over. Nerves from the temporal side of each retina go to their own side of the brain (not crossed over). Nerves from the nasal half of the retina cross over to reach the opposite side of the brain.
The brain is split into a right and left half. Consider the left visual cortex only. The visual signal comes from:
* nasal half of the right eye and
* temporal half of the left eye.
This combination is an essential first step for 3D vision, but is only half the story. The full visual field in 3D needs the right side of the brain achieving a similar combination
of signals.
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Stereoscopic vision is essential
for predator animals who catch other animals (owls for example), but
has secondary importance for animals who are chased (such as rabbits
or deer).
Both eyes seeing the same scene reduces the field of view
and allows other animals to sneak up undetected. Prey animals prefer
a panorama view of the world, where their eyes are placed on each side
of the head, allowing nearly a 360 degree field of view
(rabbits).
Humans are modified tree apes (except in certain USA states,
which deny Darwinism). Tree apes swing from branch to branch and failure
to instantly judge the world in 3D could lead to a nasty fall.
Just as with a camera, light rays cross over in the lens
so the picture on the retina is upside down and back to front. The brain
has no problem with that and just interprets the information around
the right way.
Binocular Vision 
Because our eyes are separated by about 65mm, the world
seen by each retina is slightly different. The difference is great for
objects nearby, but imperceptible for distant things, like mountains.
Useful stereoscopic vision while we are moving about fades out about 200 meters away, (1 arc minute) (In the laboratory, stereo vision can be measured at best down to 5 arc seconds: meaning stereo vision theoretically extends to 2.7Km).
In standard stereoscopic photography, useful 3d stops short of 200
meters, but can be extended far beyond by using
hyper-stereoscopic photography.
A 3D perception of the world is built up in the brain,
which computes the retinal differences and interprets them as 3D. People
looking at a stereoscopic picture may find the third dimension only
develops slowly. The longer they look, the more computation the brain
gets through and the 3D impression gets better with time. Those who
look at many 3D pictures seem to handle the computations increasingly
rapidly, so stereoscopic vision is partly a learned response. In fact,
all visual interpretation is learned in infancy, especially binocular
interpretation. The basic brain mechanisms are present from birth in
the visual cortex, but only experience leads to binocular interpretation
becoming "hardwired". If one eye has poor muscles ("cross
eyes") it is suppressed and binocular cues may never be worked
out properly, which means a squint must be treated early in childhood.
If you hold a pencil in front of your nose and look into
the distance, you see two pencils. Some people do not see two pencils
because their brains suppress information from one eye. The suppressed
eye may shift from right to left, or there may be permanent suppression
of one eye. These people may have difficulty seeing 3D pictures. People
with only one eye, or with one eye suppressed, never will see in 3D.
Components of stereoscopic vision:
-
Convergence of the eyes (achieved by the eye
muscles) until double vision is overcome.
-
Focus of the lens to render objects sharp
- Physiological diplopia. 3d vision depends on repeated corrections for double vision. As shown in the window spot experiment, double vision extends for 18 meters (see the left column.) Stereoscopic vision begins after double vision is corrected by eye movements.
Slight retinal differences, still remaining after
focus and convergence, are detected by retinal receptors which are matched
in the two eyes.
-
Big differences between retinal images are solved
by the eyes changing their convergence, to overcome double vision. (Vertical displacement and rotation can also be corrected, but only if they are small and often these errors cause discomfort.)
-
Once the brain has fused an object into one, the small
disparities that remain are interpreted as the 3rd dimension.
-
The limited range over which a retinal difference
is interpreted as 3D rather than double vision is called Panum's
fusional area.
- Even if there is double vision, the brain can integrate
the various Panum areas to produce a 3D impression,
but it takes time to achieve it. So do not rush looking at a stereo
pair, especially if there is great depth. Give the brain and eyes
time to scan across and into the stereo picture.
3D photography gives an impression of depth which is less convincing than real life.
The ability to move our heads sideways to sort out problem areas, by movement parallax, is impossible when viewing photographic stereo pairs. Better stereo images are kept relatively simple, to allow for the reduced information in still photography compared with reality.
The focus of the eyes is adjusted by changing
the shape of the lens through the ciliary muscles. As the eyes converge
to look nearby, the eyes automatically focus close to prevent the world
becoming a blur. The eyes work on "auto focus", just like
a modern camera.
But focus does not give 3D information. If the convergence
information about distance does not agree with the sharpness of the
image, the lens changes shape independently, until the blur is
removed. Focus is involuntary, and some people have difficulty with it.
Stereo pairs are always presented on a flat surface and so focus
information and binocular information are never in agreement, which
is a defect of stereoscopic displays.
We interpret the scenery as one image, which is referred
to as the "cyclopean eye view of the world". This conceptual
cyclopean eye sits in the middle, between our two real eyes.
Neuro-physiologists have found retinal differences between
the two eyes are directly mapped in the brain.
The occipital visual centres detecting retinal disparity between the
eyes are closely linked to neural centres which control eye convergence
and focus, by directing the eye muscles.
The brain is tolerant of small differences between
the two eyes. Even small magnification differences and small angles
of tilt are handled, without conscious double vision. Differences in brightness
are handled easily. Differences in contrast are interpreted as changes
in reflection angle, producing a sheen on surfaces. Differences in colour
are no problem, in fact one eye can see red and the other blue/green
and still interpret the scene as three dimensional (as in anaglyphs).
The eye has a tiny area of maximal acuity (the
fovea or macula on the retina). The fovea is devoted to high resolution
and colour reception.
The background image on this page shows the macula of the author's eye. It is the dark, central area in the retinal photograph. Note how blood vessels come close to the macula but do not cross it, producing a radial pattern. This means the steadily narrowing blood vessels do not interfere with central vision. (False colour image to enhance blood vessels. Original retinal photograph by Bryre Murphy.)
Good stereo discernment depends on good vision
because the differences between each eye image can be subtle. Keep your
correcting glasses on when viewing in 3d.
3D is seen with reduced accuracy away from the fovea,
in the more peripheral vision. Here there are colour-blind receptors (rods)
wired up to specialise in detecting differences over time rather
than spatial resolution. Movement is detected by changes in our
peripheral vision, which is essential to avoid predators (and
motor cars). All we know is something in the corner of our visual field changed. Then our eyes
turn to find out what changed, by putting it on the fovea, to judge it
in high resolution 3D.
Stereo vision does not depend on recognition of objects by each eye. Random dot stereograms reveal 3D is seen first and recognition of objects comes second. However, the world is not all random dots and in the real world we do recognise objects first and see 3D later, even if the more slowly developing 3D impression does not depend on object recognition.
Visual Integration
The eyes are connected to 6 other nerve centres in the
brain. For example, the balance system. The position of the
head is monitored by semicircular canals in the ears. The head and eyes
are turned to correct for changing head position, as measured by the
ears. If information from the eyes does not match positional data
from the ears, some people become nauseated (motion sickness)
and their eyes flick from side to side (nystagmus), trying to make the
conflicting information coherent.
Movement stereo
People with one eye can still get a 3D impression of the world by moving their heads sideways and noticing how objects move in relation to each other. Close objects move, distant objects seem to stay still.
People with two eyes also use this trick, which is well illustrated by looking out the side window of a car or train.
One of the deficiencies of stereoscopic photography is the inability to use head movements to resolve problems of 3D perception. A good example is yacht masts in a marina. Often a mast will vanish behind a closer mast in one eye but not the other. That is resolved by slightly moving the head sideways, but on a stereoscopic photograph the "mast illusion" cannot be solved.
Cross your eyes or go wall eyed to fuse this image. You will see a combination of stereoscopic and double vision, which work together as your eyes move about to build up a 3D impression.
The black dot is a problem, it is often seen as double, when the tip
of the red line is single. When the brain is confused, perspective rules
come to dominate over binocular rules of 3d. The confusion is worse
with X stereo than with U stereo |
Birds need both:
- stereoscopy, when landing on a branch and
- a panoramic view, to avoid predators.
Except for owls and raptors, bird's eyes are on opposite sides of the head, for
a wide angle view.
Looking at a bird from the front, it is usually possible to see parts
of both eyes. In this limited angle, which is common to both eyes, can birds
see in 3D?
Some birds can, but ordinary chooks cannot see straight ahead. Fowls cock their heads to one side and bob about. This provides movement depth perception for locating that tasty morsel they wish to peck.
The 3D vision part of the human retina is split vertically and on different sides for each eye. The nasal half view of one eye combines with the temporal half view of the other.
One finger test
Hold up one finger about 30cm away and then look at a distant tree. If you gaze at the tree you will seem to have two fingers. Look at the finger and there will seem to be two trees.
This is double vision or diplopia.
Now put a spot on your house window and look through the window, past the spot, at a distant tree. Move back until you only see one window spot when looking at the tree. An ordinary room is not long enough to prevent you seeing two spots.
This means in every-day life we normally see double, but are not aware of it.
You only see one window spot when you move 18 meters back from the window. Once fusion occurs, the angle between the eyes, measured at the spot, is 12 arc minutes. Or 12/60 = 0.2 of a degree.
Beyond 18 meters, the eyes are operating in Panum's fusional zone.
In Panum's zone we can still see there is a difference. The spot seems in front of the trees. This is stereo vision. Stereo vision does not cut out completely until the spot is 2.6km away. (0.000138 degrees, 5 arc seconds.) Stereo acuity varies with measurement techniques and is often lower for parallel stereo (U stereo) at 14 arc seconds. ( 900 meters). These laboratory results are not matched in ordinary vision where stereo seems to become rather useless at 200 meters away. (1 minute of arc). Stereo photographs do not give useful 3D beyond about 200 meters, unless hyper-stereoscopy is used.
The small Panum zone, of 3D without double vision, moves about as we flick our eyes between objects. We get great stereo just in front and just behind an object we are currently fixating on.
If you manage to gaze fixedly at an object, gradually 3D impression and even vision itself fades. Vision depends on our eyes flicking about (saccadic eye movements). Our field of sharp vision is so small (at the macula) saccadic eye movements are also necessary to have a clear idea of our surroundings, but it is a brain-computed clear idea and not simultaneous all over the field of view.
Do not be surprised if you see double when looking at a stereo pair. In fact, if you do not see double there is something wrong, but photographers often help you by making scenery grade slowly into the distance. This gradual shift does not seem to cause so much double vision, although that is an illusion. You mainly see double if an object nearby sits beside an object far away so they are not in the same Panum zone.
Some stereo books say double vision stops at 2 meters and stereo photographs should only be taken of objects lying 2 meters to infinity from the camera, to avoid diplopia. Certainly the diplopia is not so obvious after 2 meters, but the window spot test shows those who think double vision has stopped beyond 2 meters are deluded.
Three finger test
Hold three fingers in front of your nose. This blocks out the nasal visual field of both eyes. (Because light rays cross over in the lens of the eye, the nasal visual field is actually seen by the temporal half of the retina)
You still see a wide-angle view of the world. Move your fingers nearer or further until there is no gap in the world. The brain has fused the two nasal halves of your visual field into one panoramic view.
BUT
- you can no longer see in 3D. Stereoscopic vision has turned off. Near objects fuse with distant objects, they are all in the same plane.
- you no longer have double vision between near and far objects.
Diplopia and stereo vision work together,
until over 18 meters away when stereo works by itself,
until 200 meters away when stereo vision fades out.
Two hand test
Place your flat, opened left hand vertically on your nose and angle it left until your left eye only sees the left half of your visual field. (The temporal half.) (Remember, light rays cross over as they pass through the lens and it is actually the nasal half of the retina which is being stimulated).
Now place your flat right hand on the right side of your head and angle it until your right eye only sees the left half of the visual field. (The nasal half)
Open both eyes. You cannot see to the right, but as you look to the left, your vision is perfect 3D.
These two tests show 3D vision needs the nasal half of one eye and the temporal half of the other eye combined to make 3D.
If you only have the two nasal halves of the visual field (3 finger test), panorama vision is maintained but stereoscopic vision is lost.
Although the world detected by our visual system is "a game of two halves," the brain fuses them together to make a single wide view of the world. |
Chromostereopsis
Binocular vision is not only useful for panoramic and stereoscopic views of the world. Microscopists have long known that visual acuity, contrast and brightness is enhanced if two eyes are used rather than one. Astronomers find the same thing when viewing paired astronomical photographs in a stereoscope. An amazing binocular telescope, made in New Zealand by Dave Moorhouse, gives enhanced views of the heavens in real time. Stellar astronomy has no real stereoscopy (the stereo base is several AU too short) but stars and nebulae look better when binocular information is integrated in the brain. (3D nebulae on this web site are simulated 3D, but the stereo views of the sun are true 3D.)
The huge astronomical binoculars do give a false impression of 3D due to red and blue rays being focused differently in the eyes, as clearly described here and chromostereopsis is well seen in star clusters containing red and blue stars, such as the Jewel-box cluster (Kappa crucis).
ChromaDepth
ChromaDepth® glasses have micro-optical prisms which deviate colours differently to produce stereo parallax based on colour. Since rainbows are caused by the same process, it is no surprise that the stereo depth follows the colours of the rainbow (ROYGBIV). Red comes forward and blue moves back. Underwater photography is the example quoted for a natural ChromaDepth image, since objects further away become bluer.
Colour rivalry
The eyes are continually scanning:
flicking across the scenery. (Saccadic eye movements).
This means the
convergence points of the two eyes are continually moving, as a three
dimensional scene is viewed.
The monochromatic nature of 3D vision explains
how red/green or red/blue glasses allow a 3D impression when we look
at anaglyphs, or go to those stereo movies which depend on the
audience wearing coloured glasses.
People who are red/green colour blind can
usually see red/green anaglyphs. The function of the coloured filters
is to separate the monochrome information into two channels and the
actual colour does not matter.
(We will see later how anaglyphs can seem
to have true colours, when viewed without coloured glasses.)
Stereoscopic vision is complex:
about 2/3
of the population do not see 3D in stereoscopic photographs
1/3 can be taught and
1/3 never
see in 3D. (Information from the Philips company researching stereoscopic fluoroscopy and why it was not a commercial success)
Check your 3D vision here
There are four main problems.
- The eye muscles must give accurate information
about where the eye is pointing (proprioception)
- The eyes must be aligned correctly, with the optical axes parallel when looking into the far distance.
- The brain must not suppress the image from
one eye.
- Infants must have the chance to see properly and play simultaneously, so the 3D impression from bumping into things and manipulating them can combine with binocular visual information.
Often the double vision
which results from poor eye muscles is intolerable and the only way
the brain can handle it is to suppress one eye. If that happens in infancy, requirement 4 is not met.
When suppression happens in infants, it may
become permanent and they may never learn to see in 3D.
Recently it has been discovered things are not quite so hopeless as once thought and after good surgery, good glasses and a teaching program, binocular vision can be restored in children older than 5. See here
Binocular vision problems in children can
often be restored with prism
lenses. Symptoms caused by impaired binocular vision include mixing
up the letters d and b, transposing letters, slow reading, poor writing
and headaches. Prism lens correction even as low as 2cm/meter (which
was previously regarded as too small to worry about) is said to help
these symptoms in about 2/3 of affected children. Even attention deficit
disorders (ADD), especially in children who hold their heads at an angle
while attempting to correct distorted vision, are alleged to be helped
by prism lenses.
GO TO PAGE 2 |
Eye Exercises
Eyes at rest do not always line up the same. You can check
this by looking at an object while holding one hand in front of
one eye. Rapidly flick the hand sideways to block the other eye, uncovering
the first. You may see objects jumping, tilting, or even going out of
focus. If you wear glasses, this maneuver may reveal errors in the
prescription, especially if the magnification or focus changes. When
both eyes work together, such differences between the two are corrected
by the eye muscles.
Viewing pictures in 3D is not harmful to normal people
(but special conditions apply for children with eye problems). Some
people may get headaches at first, but the eye exercises involved in
normal stereoscopic perception are mild and may even be beneficial for
those with weak eye muscles. Any tendency to headaches disappears with
experience. Sleep cures the symptoms, so beginners should practice 3D
vision before going to bed. Literature
on binocular therapy
Headaches and even nausea are worse with cross eye stereo
(X stereo).
Well aligned parallel eye stereo, using lenses which
place the focus at infinity (weak magnifying glasses) relaxes the eye
muscles. Looking at U stereo can be a good exercise for eyes tired at work by
prolonged convergence onto a computer screen. Nearly, but not as good, as
taking the dog for a walk in twilight, which also relaxes the eyes and
neck muscles. Unfortunately, anaglyph viewing is not as relaxing as U stereo, since the eyes are not brought to full parallel and the coloured glasses are not that comfortable. Anaglyphs are a way to show that 3D is possible, they are not the ideal viewing method.
Children
who are cross eyed SHOULD
NOT ATTEMPT CROSS EYE STEREO. They are prescribed eye exercises designed to diverge the eyes. They should only use parallel eye stereo
on this web site. Get advice from your ophthalmologist / optometrist first. A good mirror viewer is recommended (such as the Screen
Scope) to help such children. Note that full parallel stereo with the images on this web site and a Screen Scope needs an 800x600 pixel screen, which you can modify as necessary in "screen properties" or Mac Preferences.
Persistence of vision
An excellent method for 3D viewing on a computer screen
depends on glasses which rapidly change from clear to black. This can
be arranged so the computer shows the left eye picture while the left
eye glass is clear but the right is black and vice versa. If
this happens fast enough (e.g. 85Hertz) there is no perceptible flicker.
The brain is fooled and extracts the 3D information as usual. No headaches,
because the eyes are functioning as they normally do when looking at
a computer.
Flicker fusion rate is high in bright areas of a picture,
and flicker is best seen with averted vision on a CRT monitor. Fusion frequency
in dim lights can fall as low as 10 cycles per second. Look up and see
if the computer is flickering in the corner of your eye, for example
in the white cartoon to the left. (CRT = Cathode Ray Tube - the traditional
big glass bottle used for computer and TV screens). TFT (LCD) computer
screens do not produce so much flicker, even if they are brighter, since
they do not turn on and off so abruptly as CRT. Some people find TFT screens cause a burning sensation in the eyes and headaches.
The author had this problem after changing to a lovely
22 inch LCD screen at work, but discovered the cure was to avoid big
white areas, such as word processing documents in black print on a white
screen. Change the Windows defaults so the white background turns to
a darker colour (I prefer light brown) and also change the screen font
to a bigger size.
(This subject was researched in case 3D viewing on
TFT screens might cause eyestrain, but the problem with LCD is unusually
fine detail displayed at remarkably high contrast on a large very bright
surface. This causes glare combined with eye focus problems, which are
prolonged over an 8 hour working day, resulting in eye burning pains
and headaches. People often screw up their faces to see the fine print, causing muscle ache or headache. Most stereoscopic pictures are not high contrast, fine detail and do not cause this problem).
Advice
on prolonged computer use from Keystone company. Not quite the same
advice as here, but make your own judgment. They suggest black print
on a white screen is good - I disagree rather strongly about that.
Flicker fusion shows the eyes are turning on and off,
which is controlled in the retina itself. Small insects have a
very high fusion frequency of around 300 hertz, which is why it
is hard to catch a fly, especially on a summer day when its muscles
are warmed up. Our hand crashing down in a blur is seen by the fly as
a large number of static images, giving plenty of time to fly off, by
a direct reflex from the eye to the wing muscles, bypassing what little
pathetic brain the fly might have.
Flicker induced epilepsy
Liquid crystal glasses used for 3D have caused epilepsy
in susceptible people. Flickering
lights can trigger an abnormal zone in the brain, where nerve
impulses circulate in a loop. If the time for impulses to circle the
aberrant nerve loop is close to the light flicker frequency, the feedback
loop resonates and starts a seizure. CRT television, Video games and
disco lights are other causes of this. Flickering below 5 hertz and
over 80 cycles per second is usually OK. Big light areas cause more
trouble, which is one of the reasons for not using a white background
for web pages, although computer screens usually refresh so fast there
is less trouble than with TV. LCD screens flicker less because
they do not turn off so fast as CRT screens.
Difficulty with 3D movies
Modern stereo movies, like Avatar, have exposed the 15 to 20 percent of people with binocular fusion difficulty because they end up with:
- Nausea
- Vomiting
- Headache
- Inability to stay awake.
further information |
